An invasive crab in the South Atlantic Bight: friend or foe?

Although the South Atlantic Bight has experienced few invasions of invertebrate macro-fauna, the green porcelain crab, Petrolisthes armatus, recently invaded oyster reefs at densities of thousands m. We assessed the spatial and temporal dynamics of P. armatus densities, sex ratios, sizes, and reproductive patterns via a large-scale monitoring program in two estuaries of coastal Georgia, U.S.A. Exotic crab densities generally were higher in the low versus the high intertidal and in the lower versus upper regions of estuaries. In warmer months, 20 to 90% of adult females were gravid, recruits were dense, and population levels were high. In colder months, densities dropped by 64 to >99%. Male:female ratios were near 1:1 across times and locations. Native mud crab densities were unrelated to, or positively correlated with, densities of exotic crabs; correlations were never significantly negative. These correlations did not lend support to the hypothesis that the invasive crab is negatively affecting native crabs with which it most closely associates. Maximum mean densities of P. armatus in Georgia were 37 times the highest densities recorded in the historic range, crabs in the new range reproduced at a smaller size, and the percentage of gravid females was similar between the old and new range. Thus, population fecundity in the new range exceeds that of the native range by more than an order of magnitude. The effects of high density, rapidly